Dynamic data restore in thyristor-based memory device

ABSTRACT

A dynamically-operating restoration circuit is used to apply a voltage or current restore pulse signal to thyristor-based memory cells and therein restore data in the cell using the internal positive feedback loop of the thyristor. In one example implementation, the internal positive feedback loop in the thyristor is used to restore the conducting state of a device after the thyristor current drops below the holding current. A pulse and/or periodic waveform are defined and applied to ensure that the thyristor is not released from its conducting state. The time average of the periodic restore current in the thyristor may be lower than the holding current threshold. While not necessarily limited to memory cells that are thyristor-based, various embodiments of the invention have been found to be the particularly useful for high-speed, low-power memory cells in which a thin capacitively-coupled thyristor is used to provide a bi-stable storage element.

RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 11/361,334,filed on Feb. 24, 2006, now U.S. Pat. No. 7,405,963, which is acontinuation of U.S. patent application Ser. No. 11/112,090, filed onApr. 22, 2005, now U.S. Pat. No. 7,042,759, which is a continuation ofU.S. patent application Ser. No. 10/472,737, filed on Sep. 22, 2003, nowU.S. Pat. No. 6,885,581, issued on Apr. 26, 2005, which is a nationalunder 35 USC §371 of PCT/US02/10706, filed on Apr. 5, 2002, which claimspriority and benefit to U.S. Provisional Patent Application Ser. No.60/281,893, filed on Apr. 5, 2001, to which priority is claimed under 35U.S.C. §120 for common subject matter, each of which is incorporated byreference.

This application relates to U.S. patent applications Ser. No.09/814,980, filed on Mar. 22, 2001, now U.S. Pat. No. 6,462,359, issuedon Oct. 8, 2002; and Ser. No. 09/815,213, filed on Mar. 22, 2001, nowU.S. Pat. No. 6,727,528, issued on Apr. 27, 2004, both of which areincorporated by reference.

FIELD OF THE INVENTION

The present invention is directed to semiconductor devices and, morespecifically, to semiconductor memory devices. The present invention hasbeen found to be particularly advantageous for memory devices includingthyristor-based memory cells.

BACKGROUND

Recent technological advances in the semiconductor industry havepermitted dramatic increases in integrated circuit density andcomplexity, and equally dramatic decreases in power consumption andpackage sizes. Presently, single-die microprocessors are beingmanufactured with many millions of transistors, operating at speeds ofhundreds of millions of instructions per second and being packaged inrelatively small, air-cooled semiconductor device packages. Theimprovements in such devices have led to a dramatic increase in theiruse in a variety of applications. As the use of these devices has becomemore prevalent, the demand for reliable and affordable semiconductordevices has also increased. Accordingly, the need to manufacture suchdevices in an efficient and reliable manner has become increasinglyimportant.

An important part in the design, construction, and manufacture ofsemiconductor devices concerns semiconductor memory and other circuitryused to store information. Conventional random access memory devicesinclude a variety of circuits, such as SRAM and DRAM circuits. Theconstruction and formation of such memory circuitry typically involvesforming at least one storage element and circuitry designed to accessthe stored information. DRAM is very common due to its high density(e.g., high density has benefits including low price), with DRAM cellsize being typically between 6 F² and 8 F², where F is the minimumfeature size. However, with typical DRAM access times of approximately50 nSec, DRAM is relatively slow compared to typical microprocessorspeeds and requires refresh. SRAM is another common semiconductor memorythat is much faster than DRAM and, in some instances, is of an order ofmagnitude faster than DRAM. Also, unlike DRAM, SRAM does not requirerefresh. SRAM cells are typically constructed using 4 transistors and 2resistors, or 6 transistors, which result in much lower density, withtypical cell size being between about 60 F² and 150 F².

Various SRAM cell designs based on a NDR (Negative DifferentialResistance) construction have been introduced, ranging from a simplebipolar transistor to complicated quantum-effect devices. These celldesigns usually consist of at least two active elements, including anNDR device. In view of size considerations, the construction of the NDRdevice is important to the overall performance of this type of SRAMcell. One advantage of the NDR-based cell is the potential of having acell area smaller than four-transistor and six-transistor SRAM cellsbecause of the smaller number of active devices and interconnections.

Conventional NDR-based SRAM cells, however, have many problems that haveprohibited their use in commercial SRAM products. These problemsinclude, among others: high standby power consumption due to the largecurrent needed in one or both of the stable states of the cell;excessively high or excessively low voltage levels needed for celloperation; stable states that are too sensitive to manufacturingvariations and provide poor noise-margins; limitations in access speeddue to slow switching from one state to the other; limitations inoperability due to temperature, noise, voltage and/or light stability;and manufacturability and yield issues due to complicated fabricationprocessing.

A thin capacitively-coupled thyristor-type NDR device can be effectivein providing a bi-stable element for such memory cells and in overcomingmany previously unresolved problems for thyristor-based memoryapplications. This type of NDR device has a control port that iscapacitively coupled to a relatively-thin thyristor body. The thyristorbody is sufficiently thin to permit modulation of the potential of thethyristor body in response to selected signals capacitively coupled viathe control port. Such capacitively-coupled signals are used to enhanceswitching of the thyristor-based device between current-blocking andcurrent-conducting states.

An important consideration in the design of thyristor-based memorycells, including the above thyristor-based type, concerns maintenance ofthe thyristor's conducting state. When the thyristor is in the forwardconducting state, a DC current larger than the holding current of thethyristor flows through the thyristor in order to maintain theconducting state. For the specific case of the above thyristor-basedtype memory cell, optimal operation of the device is challenged byvarious issues. For example, when a MOSFET access transistor is used tocontrol the current flow through a thyristor, variations in thethreshold voltage of access transistors from cell to cell in a largearray and the exponential current-voltage dependence for accesstransistors in the sub-threshold regime can result in an unduly largestandby current for the array and/or loss of the conducting state forsome of the thyristors in the array. In addition, unduly large standbyresistors often can be used in each memory cell. This resistor can addto the bit-cost of the memory by adding some extra steps to thefabrication process and potentially increasing the memory cell size.Furthermore, the resistance variation of the standby resistor used fromcell to cell can result in a large standby current for the array and/orloss of the conducting state for some of the thyristors in the array.Other related challenges include variations in bit-line voltage duringread and write operations into one cell and a resultant large standbycurrent and/or loss of the conducting state for the other cells sharingthe same bit line.

SUMMARY

The present invention is directed to overcoming the above-mentionedchallenges and others related to the types of devices discussed in theabove-indicated related applications and in other memory cells. For morespecific examples of these devices to which the present invention isapplicable, reference may be made to each of the above-mentioned patentdocuments and to the publication cited therein and in the prioritypatent document, each of which is incorporated by reference in itsentirety.

Generally, the present invention is directed to dynamic data restorationin a memory device having an array of memory cells and with each memorycell having an internal positive feedback loop. A restore current orvoltage pulse is applied to each memory cell for a short interval. Thepulses can be applied periodically with each applied pulse defined torestore a forward conducting state of an element in the memory cell inresponse to the internal positive feedback loop.

According to one aspect, the present invention is directed to a methodfor dynamically restoring data in a thyristor-based memory device, suchas a memory cell array, having a plurality of thyristor-based memorycells. In each memory cell, a thyristor with an internal positivefeedback loop is used to provide the storage element. The methodincludes applying a current or voltage restore pulse for a shortinterval to each memory cell and therein restoring data in the cellusing the internal positive feedback loop of the thyristor.

According to an example embodiment of the present invention, arestoration circuit is used to apply a voltage or current pulse orwaveform to the thyristor of a thyristor-based memory cell and thereinrestore data in the cell using the internal positive feedback loop ofthe thyristor. In one implementation, the internal positive feedbackloop in the thyristor is used to restore the conducting state of adevice after the thyristor current drops below the holding current. Thepulse waveform and frequency are defined to ensure that the transistoris not released from its conducting state. This restoration is typicallyapplied after the thyristor device is fully in the forward conductingstate and in a manner that prevents the thyristor device fromtransitioning completely out of the forward conducting state.

The above summary of the present invention is not intended to describeeach illustrated embodiment or every implementation of the presentinvention. The figures and detailed description that follow moreparticularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thedetailed description of various embodiments of the invention inconnection with the accompanying drawings, in which:

FIG. 1 shows a semiconductor device including an array ofthyristor-based memory cells, according to an example embodiment of thepresent invention;

FIG. 2 shows the I-V characteristics of a thyristor and two parametersof the thyristor used in connection with an example embodiment of thepresent invention;

FIG. 3A shows an example arbitrary periodic current waveform forrestoring the conducting state of a thyristor, according to the presentinvention;

FIG. 3B shows an example standby current waveform adapted to maintainthe conducting state of a thyristor, also according to the presentinvention;

FIG. 4 is a block diagram of a control circuit for a thyristor-basedmemory cell, according to another example embodiment of the presentinvention; and

FIGS. 5 and 6 are timing diagrams, each timing diagram respectivelyshowing operations of a thyristor-based memory device using a dynamicstandby current scheme, according to two respective example embodimentsof the present invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not necessarily to limit the invention tothe particular embodiments described. On the contrary, the intention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

The present invention is believed to be applicable to different types ofmemory devices, and has been found to be particularly useful for suchdevices using high-speed, low-power, thyristor-based memory cells. Whilethe present invention is not necessarily limited to such devices,various aspects of the invention may be appreciated through a discussionof various examples using this context.

According to an example embodiment of the present invention, a dynamicstandby current is used to restore the forward conducting state of athyristor device. This restoration is after the thyristor device isfilly in the forward conducting state and before the thyristor devicetransitions into the current blocking state. In this context, it hasbeen discovered that the forward conducting state of a thyristor devicecan be restored (i.e., maintained or reinstated) using the internalpositive feedback loop in the thyristor. The junctions of the thyristorin the forward conducting state are saturated with minority carriers,and it takes a relatively long time for all the minority carriers torecombine, once the thyristor current is cut off. In one exampleimplementation, the internal positive feedback loop in the thyristor(for example, by two back-to-back connected PNP and NPN bipolartransistors, as in FIGS. 3 a and 3 b in U.S. Pat. No. 6,229,161), isused to restore the conducting state of the thyristor after its currentis cut off. The current waveform is periodically pulsed for thethyristor and the forward conducting state of the thyristor ismaintained in the steady state.

FIG. 1 shows an array of thyristor-based memory cells, such as thosediscussed above, along with an array-access controller 102, according toa specific example embodiment of the present invention. The array-accesscontroller 102 includes a decoder 104 for decoding bit lines and wordlines used to address and access the memory cells. As will be discussedfurther below, the array-access controller 102 also includes arestoration circuit 106 that is used to help maintain or reinstate aforward conducting state.

Each cell, a representative one of which is depicted as 108, includes aPNPN-type NDR device, shown as thyristor 110, and a storage-node accesscircuit that is exemplified by a MOS-type pass (or access) transistor112. The transistor 112 includes a gate 114 that forms part of a firstword line (WL1) 116, and is used to provide electrical coupling betweena bit line (BL) 118 and a data storage node 124 for the cell 108. Thus,the lower one of the drain and source regions of the transistor 112 isconnected to BL 118. At the top of the device 110 is a node 120 that iscommonly shared by each cell for connecting the top terminal of thedevice to a supply or reference voltage, Vref. Fabrication of each celland its components can be implemented using any of various alternativeapproaches; for example, the device 110 in each cell can be made: over aportion of the access transistor; over the source or drain that is notconnected to the bit line; adjacent to the access transistor; in asilicon-on-insulator (SOI) application; in more traditional siliconsubstrate (non-SOI) application; and in other forms such as thosedescribed in the above-referenced patent documents.

In each cell of the array, the device 110 has a middle region that isadjacent to a charge plate, depicted as gate 122. The gate 122 formspart of a second word line (WL2) 117 and is used to enhance switchingbetween the cell's two stable states: the OFF state, where the device110 is in a current-blocking mode; and the ON state, where the device110 is in a current-passing mode. These two states are used to controlthe storage node 124, shown interconnecting the lower terminal of thedevice 110 with the upper source/drain region of the transistor 112. Thevoltage of the storage node 124 is at its high value for the ON state.

Generally, as described in the above-mentioned U.S. patent, typicaloperation of the cell 108 involves using the array-access controller 102to provide appropriate control over the bit line and word linesconnecting to the cell 108. For example, in standby mode, the word linesand the bit line are inactive or at their low voltage levels (which canbe different for each line). For a write “Zero” operation, BL 118 israised to its high level and WL1 116 becomes active. This charges thelevel at the storage node 124 to a high voltage level and moves thedevice 110 out of the strong forward biased region. A pulse is thenapplied to WL2 117. Capacitive coupling from WL2 117 to the adjacentbody region of the thyristor device 110, e.g., where the adjacent bodyregion is the middle P-doped region of the PNPN thyristor, results in anoutflow of the minority charges from the middle P-doped region of thePNPN on the falling edge of the WL2 pulse and blocks the current pass.The device 110 is sufficiently thin so that the gate 122 has tightcontrol on the potential of the body region, and can modulate thispotential by the capacitive coupling. The device 110 is switched to theblocking state after this operation.

For a write “One” operation, the voltage level of BL 118 is held low.After WL1 116 is raised to its high level, a pulse is applied to WL2117. The rising edge of this pulse raises the potential of the P regionby capacitive coupling and makes the NP and lower PN junctions forwardbiased which, in-turn, starts the well-known regenerative process in thePNPN thyristor construction and the device 110 transitions to itsforward conducting state. After completing such an operation, controlover the bit and word lines typically changes to effect the standby modein which a current path through the transistor 112 is blocked.

As mentioned above, the array-access controller 102 also includes arestoration circuit 106 that is used to help maintain or reinstate theforward conducting state for a thyristor-based RAM cell, such as withthe above example of FIG. 1, after current passing through the thyristordevice is removed. The restoration circuit 106 and the array-accesscontroller 102 provide the bit-line and word-line control to each cellof the array as necessary for adequate standby current to restore theregenerative positive feedback effect in the thyristor.

In the example case of the specific cell array of FIG. 1, this controlcan be readily implemented in a variety of different ways. For example,in one such implementation, the pass (or access) transistor 112 of FIG.1 is periodically turned on for a short time interval to allow currentinto the bit line, which is kept at its standby low level. The accesstransistor can then be completely shut off until the next period. If thethyristor is holding a “Zero”, i.e., if the thyristor is in the forwardblocking state, the periodic turn-on of the access transistor does notflow any current. If the thyristor is holding a “One”, i.e., if thethyristor is in the forward conducting state, the periodic turn-on ofthe access transistor results in a rather large voltage drop (˜0.6V)across one of the base-emitters of the thyristor, which restarts theregenerative positive feedback effect in the thyristor and flows currentinto the bit line for a short time interval that restores the “One”state. As mentioned above, the time average of this current over aperiod can even be smaller than the I_(H) of the thyristor to maintainthe conducting state. Using this approach, data stored in thethyristor-based RAM cell (either “One” or “Zero”) is maintained in thesteady state while the average standby current of the cell can besmaller than the holding current of the thyristor.

The periodic turn-on of the access transistor can be controlled eitherby periodically pulsing WL1 or by periodically pulsing BL. Bydynamically controlling the standby current, the sensitivity of thestandby current of the cell to threshold voltage variations of theaccess transistor, variations in I_(hold) of the thyristor and bit-linedisturbs during the read and write operations for the adjacent cells isreduced or completely eliminated because the access transistor can becompletely shut off when the cell is in the idle mode (meaning thatthere is no read, write, or dynamic standby current for the cell and WL1is not active).

Various implementations of the dynamic standby current control in athyristor-based memory can be used to address challenges to memoryapplications, such as DRAM applications, including those discussedabove. For example, the contents of the thyristor-based memory are notnecessarily required to be sensed to restore the conducting state of thecell after a restore operation, and the charge restoration is very fast.In addition, multiple cells per each bit line can be restored at thesame time and the standby current control pulses in a thyristor-basedmemory can be effected within the array cycle time. In case of DRAM, thedynamic refresh operation does involve a sense circuit. Also, in suchimplementations, typically only one cell per bit line is sensed in thememory circuit at a given time.

In connection with the present invention, it has also been discoveredthat the forward conducting state of a thyristor device can be restoredusing a current waveform that has an average amplitude over a given timethat may or may not be larger than the current otherwise needed to“hold” the thyristor in the forward conducting state; also known as the“holding current” for the thyristor (“I_(H)”). FIG. 2 shows the I-Vcharacteristics of a thyristor and two parameters of the thyristor,which are forward break-over voltage (V_(FB)) and holding current(I_(H)), for use in connection with another example embodiment of thepresent invention.

FIG. 3A shows an example periodic current waveform for the thyristorwhose time average is either smaller, equal, or larger than I_(H); thus,various alternative implementations of the present invention generateand apply such a periodic waveform for the thyristor with the waveformhaving a time average that is either smaller, equal, and larger thanI_(H). In this context, the relationship between the time average andthe holding current (threshold) is as follows:

${\frac{1}{T}{\oint\limits_{T}{{I_{T}(t)} \cdot {t}}}} \leq \geq I_{H}$

The example current waveform shown in FIG. 3A is selected to maintainthe conducting state of the thyristor.

FIG. 3B shows a standby current waveform that is generated and appliedto maintain the conducting state of a thyristor, according to anotherexample embodiment of the present invention. In this example, a periodiccurrent waveform is generated and applied to flow 100 μA for 1 nsthrough the thyristor; the waveform has a period of 100 μs or less iscapable of maintaining the conducting state of the thyristor.

In another example embodiment, the frequency and the amplitude of thestandby current pulses in a thyristor-based memory are adaptivelycontrolled to track the variations of some device properties. Forexample, variations in the holding current of the thyristors or theleakage current of the access transistors can be tracked withtemperature. These variations can be tracked, for example, from wafer towafer, die to die, and even separately for different blocks of memory onthe same die.

FIG. 4 shows a block diagram of a circuit adapted to adjust thefrequency and/or amplitude of a thyristor based memory cell and to trackone or more specific variations in the holding current threshold of arepresentative (e.g., “dummy” or spare) cell 402 in the memory device(which contains the array of memory cells), according to another exampleembodiment of the present invention. In a more specific implementationof FIG. 4, the specific variation is temperature, and the representativecell 402 is located in close proximity to the cells that it represents.

For such implementations, if the representative cell 402 were to fail inmaintaining its thyristor in its conducting state after the current isremoved, the voltage of its storage node (SN) would gradually drop to alevel below the level generated by the voltage reference circuit 404(which also feeds Vref at the top terminal of the thyristor). Forexample, if this level were to gradually drop from about 0.8V to about0V, this drop would result in the output of the comparator 408 goinghigh and, via feedback per a level-shifting circuit 406, in a “One”being written back to the representative cell 402. In turn, this actionwould raise SN and again lower the output of the comparator 408.

The pulse generated at the output of the comparator 408 is integrated atintegrator 412 to create a quasi-DC voltage that is used to control thefrequency of the periodic standby current pulses. In this illustratedexample, the integrator 412 outputs a control signal used to control amodulation circuit 414, such as a voltage-controlled oscillator (VCO),which provides the frequency of the periodic standby current pulses.

These periodic standby current pulses are fed back to access (or pass)transistor within the representative cell 402 via logical “OR” gate 416.Alternatively, the modulation circuit 414 is an amplitude modulator or apulse-width modulator that controls the amplitude and/or width of thecurrent pulses for the restoration associated with the representativecell 402.

By using a circuit, such as the circuit shown in FIG. 4, as part of orin addition to an array-access controller, such as the array-accesscontroller 102 shown in FIG. 1, a restore pulse and/or a waveformincluding the restore pulse is readily defined to provide theabove-discussed standby current.

FIG. 5 shows the operation of a thyristor-based memory device using adynamic standby current scheme, according to another example embodimentof the present invention. The timing of the standby current cycles iseffected, for example, by pulsing a number of cells per each bit lineand/or word line for a short time at the end of every read or writecycle. Two of these post-memory-access pulses are respectively depictedas 502 and 504 in FIG. 5.

More specifically, FIG. 5 shows example waveforms for WL1, WL2, BL, andthe thyristor current (I_(T)) for a typical cell during different Read,Write, Idle and Standby operations. Each cycle is divided into twosub-cycles, an access (read/write) sub-cycle and a restore sub-cycle.During the restore sub-cycle, BL is always at its standby level and WL1periodically receives a pulse to restore the thyristor's state; forexample, WL1 is periodically pulsed about every 100 seconds. In thisexample, the cell is Idle when it is not being accessed. During the Idleoperation, another cell sharing the same bit line can be accessed for aRead or Write operation during that sub-cycle (thus, the undefined-timeregions 506 and 508 along BL in FIG. 5).

Consistent with another example embodiment of the present invention,FIG. 6 shows a manner in which to effect the timing of the standbycurrent. As with FIG. 5, example waveforms are represented by WL1, WL2,BL, and the thyristor current (I_(T)) for a typical cell duringdifferent Read, Write, Idle and Standby operations. In this instance, afull cycle is dedicated for provision of the standby current pulse 602during which a read or write operation on the standby bit lines is notperformed. During the standby cycles, BL is always at its standby leveland WL1 is pulsed. In one example implementation, this standby currentpulse 602 is applied about every 100 seconds to restore the thyristor'sstate. In FIG. 6, the cell is “Idle” when the cell is not beingaccessed; during the Idle operation, another cell sharing the same bitline can be accessed for a Read, Write, or Standby-pulse operationduring that cycle.

According to yet another example embodiment of the present invention,the above-described approach of using the internal positive feedbackloop of a memory cell for data restoration is applied to memory cellsother than thyristor-based memory cells. For example, a memory cell thathas an internal positive feedback loop can be restored by periodicallyapplying a dynamic standby current or voltage pulse to the memory cellin a manner not inconsistent with the above-described implementations.

In another implementation, an access cycle to a thyristor-based memoryis partitioned to a read/write and restore cycle. In anotherimplementation, a complete memory cycle is partitioned to effect dynamicstandby current flow. In another implementation, the period and/oramplitude of the dynamic standby current is pulsed to track variationsof the holding current. In still another implementation, the throughputof dynamic standby current restoration of a thyristor-based memory cellarray is improved in a manner that includes simultaneously flowingstandby current pulses for more than one cell per bit line.

While the present invention has been described with reference to severalparticular example embodiments, those skilled in the art will recognizethat many changes may be made to these example embodiments. Forinstance, enhancing the standby current as described above in connectionwith one or more of these example embodiments can be supplemented usinganother standby current approach, with line control, timing and levelsadjusted as may be necessary for proper operation. Further, the skilledartisan will recognize that the “blocked” circuits and generated signals(and/or waveforms) of FIGS. 1 and 4 can be implemented per the levelsand timing of a specified application and using a variety of availablecircuits and/or circuit components, including discrete,semi-programmable and fully-programmable digital and/or analogtechnologies. These changes do not necessarily depart from thecharacterization, spirit and/or scope of the present invention, which isset forth in the following claims.

1. A semiconductor device comprising: a memory cell comprising athyristor disposed electrically in series with an access device; and arestore circuit operable to apply periodic pulses to the memory cell tomaintain its data state.
 2. The semiconductor device of claim 1, inwhich the restore circuit is operable to apply the periodic pulses to atleast one of the thyristor and the access device.